Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type.
Exhaust aftertreatment systems receive and treat exhaust gas generated by an internal combustion engine. Typical exhaust aftertreatment systems include any of various components configured to reduce the level of regulated exhaust emissions present in the exhaust gas. For example, some exhaust aftertreatment systems include any of various catalysts or catalytic convertors. The catalyst may be, or the catalytic convertor may include, an oxidation catalyst that stores oxygen. An oxidation catalyst utilizes oxygen in the exhaust gas stream to oxidize carbon monoxide into carbon dioxide and/or oxidize (e.g., burn) unburned hydrocarbons. During lean exhaust conditions (e.g., higher than a 1:1 air-to-fuel ratio), excess oxygen not used to oxidize carbon monoxide or unburned hydrocarbons can be stored on the walls of the catalyst. When lean exhaust conditions transition into rich exhaust conditions (e.g., lower than a 1:1 air-to-fuel ratio), the stored oxygen is released from the catalyst to supplement the lower quantities of oxygen in the exhaust gas stream in the oxidization of carbon monoxide or unburned hydrocarbons. Then, when rich exhaust conditions transition back to lean exhaust conditions, the oxygen released from the catalyst is replaced with excess oxygen in the exhaust gas stream as mentioned above.
Some conventional internal combustion engine systems are configured to control the combustion of fuel within the cylinders of an engine to affect a change in the air-to-fuel ratio within the exhaust gas stream generated by the engine. Fuel combustion properties typically are regulated by adjusting the amount of fuel added to intake air or charge air whether before or after the air is introduced into the cylinders. Basically, the air-to-fuel ratio of the fuel and air mixture within the cylinders corresponds with the air-to-fuel ratio of the exhaust gas resulting from the combustion event. Therefore, increasing the amount of fuel added to the intake air correspondingly decreases the air-to-fuel ratio in the exhaust gas, and decreasing the amount of fuel added to the intake air correspondingly increases the air-to-fuel ratio in the exhaust gas. Some internal combustion engines include external fuel injection strategies that include injecting fuel directly into exhaust gas to adjust the air-to-fuel ratio of the exhaust gas.
Most fuel injection strategies for adjusting the air-to-fuel ratio of an exhaust gas stream include injecting fuel into the air intake stream upstream of the combustion cylinders of the engine. The fuel may be injected solely by means of a main fuel injector in a main fuel injection event. However, because of the relatively large amounts of fuel being injected in a main fuel injection event, smaller, more precise, and more responsive adjustments (e.g., fuel dithering) to the air-to-fuel ratio in the air intake can be achieved by smaller fuel injectors positioned at various locations within an air intake system of the engine. For example, a smaller fuel injector may be positioned within the air intake system to inject fuel into the air intake stream just upstream of a turbocharger compressor during a fuel dithering event. Alternatively, or in addition, one or more smaller fuel injectors may be positioned within the air intake system to inject fuel into the air intake stream downstream of the turbocharger compressor and upstream of a throttle valve, and/or downstream of the throttle valve and upstream of an intake manifold, during a fuel dithering event. Further, for direct fuel injection engines, fuel dithering can be accomplished by adjusting the amount of fuel injected directly into the combustion cylinders.
As mentioned above, oxidation catalysts are positioned in an exhaust aftertreatment system well downstream of the combustions cylinders of the engine. Because of the relatively long distance (e.g., large volume), and high number of components, between the fuel injection sites of the main fuel injector, or dithering fuel injectors, and the oxidation catalyst, changes to the air-to-fuel ratio in the air intake stream affected by the injection of fuel are not present in the exhaust gas entering the oxidation catalyst until after a significant delay. Accordingly, the responsiveness of fuel dithering on the air-to-fuel ratio of exhaust gas entering an oxidation catalyst can be less than desirable, particularly during quickly changing transient operating conditions of the engine.
Further, potential unpredictable and inconsistent behavior of the several components between oxidation catalysts and fuel injection points may lead to unpredictable, inconsistent, and imprecise air-to-fuel ratio results at the entrance to the oxidation catalysts.
In addition to poor responsiveness, because conventional fuel dithering includes injecting additional fuel into the air intake stream upstream of the combustion cylinders, fuel dithering techniques impact the combustion events occurring in the combustion cylinders. Moreover, the impact fuel dithering techniques have on the combustion events may be negative, such as lower fuel efficiency, higher exhaust temperatures, and higher exhaust gas emissions.